The present invention relates to a tandem-type image forming apparatus in which image forming stations for respective colors are arranged along a transfer belt, each image forming station being composed of an image carrier, a charging means and a developing means which are arranged around the image carrier. The image forming apparatus forms a multi-color image by passing the transfer belt through every station.
Tandem-type image forming apparatuses as described above are categorized into two types as:
(1) an apparatus employing a paper delivery method which comprises a plurality of image forming stations arranged in an array, in which a receiving medium is electrostatically attracted to a delivery belt and is fed to be brought in contact with the respective stations in order and electrostatic transferring force is applied between each station and the recording medium, thereby superposing toner images of plural colors while directly transferring the toner images to the receiving medium; and
(2) an apparatus employing an intermediate transfer method which comprises a plurality of image forming stations arranged in an array, in which an intermediate transfer belt made of a dielectric substance is fed to be brought in contact with the respective stations and electrostatic transferring force is applied between each station and the intermediate transfer belt so as to transfer primarily toner images of the respective stations one by one to superpose the toner images on the intermediate transfer belt and the superposed toner images are transferred secondarily from the intermediate transfer belt to a recording medium at once.
In the aforementioned paper delivery method, it is required to provide a means (roller or brush) for attracting the receiving medium to the delivery belt and high voltage power supply. In the intermediate transfer method, however, such a means and high voltage power supply are not required. Further, in the paper delivery method, it is required to strictly control the transfer bias to be applied to respective image transferred portions according to the size, the thickness, and the kind of the receiving medium. In the intermediate transfer method, the primary transfer of toner images is conducted to the intermediate transfer belt of which resistance, thickness, and surface roughness are constant regardless of the aforementioned factors of the receiving medium. The control of the transfer condition including the transfer voltage or transfer current and contact pressure must be conducted only for the secondary transfer of the toner images to the receiving medium. Therefore, the intermediate transfer method has a lot of advantages.
It is required to provide a cleaning means for removing toner remaining on the surface of the transfer belt after the transfer. Normally, a back-up roller is disposed on the back of the transfer belt to oppose a cleaning blade as the cleaning means. If using a belt tension roller as the back-up roller, the reduction in size and cost of the apparatus can be achieved. However, since the position of the belt tension roller is not constant because of allowable error in length of the transfer belt and allowable variance in tension, the contact position of the cleaning blade is varied and the cleaning performance is therefore not stable.
The first object of the present invention is therefore to provide a transfer belt unit and an image forming apparatus using the same, capable of resolving the aforementioned conventional problems, which can achieve the reduction in size and cost and can prevent the cleaning performance from being varied.
In the both cases of the intermediate transfer method and the paper delivery method, there is another problem that the accuracy of color registration may be deteriorated due to meandering of the intermediate transfer belt or the paper delivery belt (hereinafter, both referred to as “transfer belt”). The meandering of the transfer belt results from difference in peripheral length between both sides of the transfer belt or variation in material of belt which may be generated in the manufacturing process, or parallelism between a driving roller and a driven roller for the transfer belt.
Though various devices for controlling the meandering of the belt have been proposed, any of them is complex in structure.
The second object of the present invention is therefore to provide a transfer belt unit and an image forming apparatus using the same, capable of resolving the aforementioned conventional problems, which can prevent the meandering of a transfer belt with a simple structure.